Switching power supplies are well known for their excellent performance over a wide range of load conditions and operating temperatures. Examples of switching power supplies include forward converter and flyback designs. Switching power supplies are typically used to convert a first DC voltage, e.g., a high input voltage, into a second DC voltage, e.g. a voltage which is relatively low. Since these power supplies typically use transformers for energy conversion, they are used in applications which require input/output isolation. An example of a switching power supply having input/output isolation is disclosed in U.S. Pat. No. 4,323,961 issued to Elliot Josephson.
As with any type of regulated power supply, an isolated power supply requires some means for sensing its output voltage to complete the regulator loop. To preserve input/output isolation, it is essential that the sensed output voltage is also fed back via an isolator circuit. Typical isolators include opto-isolators or feedback isolation transformers. In situations where a high degree of isolation is required, most prior art sensors have been quite complex and expensive.
One type of isolated sensor is described in U.S. Pat. No. 4,030,041, by Sasaki. In this device, a signal derived from the primary winding of a transformer is used to switch an optical isolator. The output of the optical isolator is used to control a FET switching device which is coupled between one secondary winding of the transformer and a buffer amplifier. A capacitor is also coupled to the input of the buffer amplifier. The Sasaki invention relies on a transformer having a primary and four secondary windings and an optical isolator to unite the operation of the device. This device has several disadvantages. For example, since the isolation transformer is embedded into the line power transformer, the bandwidth of the device is limited. Since the operation of the device is synchronized to a supplied line frequency, its operation is also relatively slow. Furthermore, switching devices which rely on optical isolators to provide isolation between primary and secondary control signals typically exhibit poor stability over time and temperature.
A somewhat simpler device is disclosed in U.S. Pat. No. 3,931,582, issued to Kato, et al. In this device, a DC amplifier supplies power to a first series coupled diode and condenser through an electronic switch which is driven by a pulse generator. This diode and condensor combination is coupled across the primary winding of a transformer. An identical second diode and condenser combination is coupled across the secondary winding of the transformer. Feedback is provided to the DC amplifier through a resistor which is coupled between the inverting input of the DC amplifier and the common terminal of said first diode and condensor combination. In this device, the active circuitry is located on the primary side of the transformer and is therefore subject to damage from any high voltage which may appear thereon. Since a DC amplifier is used to drive the primary of the transformer, the device is also subject to variations in linearity over a range of temperatures. Furthermore, this circuit is more complex than is desirable because it requires identical diode and condenser combinations on both sides of the transformer to generate an isolated voltage. Any variation in the matching between these identical circuits will affect the accuracy of the device. While this device discloses a single transformer structure, it is disadvantageous because it does not provide a controlled discharge path for the capacitor and it cannot respond to signals with fast fall times. Furthermore, the capacitor voltage cannot be held constant due to leakage through a resistor.
Other types of sensors employ devices known as choppers or synchro-rectifiers which synchronize the operation of the sensor circuitry. In this class of device, two transformers are required; one for the sensed analog signal and one for the synchronization signal. Still other chopper or synchro-rectifiers employ a single transformer having multiple secondary windings therein to operate the switching device. An example of an isolated sensor of this type is shown in U.S. Pat. No. 4,506,230 by Ashley-Rollman and in U.S. Pat. No. 4,286,225 by Morong. In each of these references, a complicated transformer structure is required. As a result, the capacitive coupling between each side of the circuit is relatively high. Furthermore, each of these references requires active components on the input side of the circuit thus requiring a relatively stable isolated power supply.
While the above discussion discloses the use of isolated sensors in switching power supplies, isolated sensors are also used in a variety of other applications. For example, isolated sensors are often used in data acquisition applications to protect computer data input lines from spurious DC signals appearing thereon. Isolated sensors are also widely used in medical applications where it is critical to protect a patient from any high voltage which may be generated by medical testing equipment.
One type of isolated sensor that is often used in medical instrumentation devices is the instrumentation grade isolation amplifier. One example of an instrumentation grade isolation amplifier is shown in U.S. Pat. No. 3,946,324 by Smith. Instrumentation grade isolation amplifiers are necessarily quite complex and as such they are among the most expensive of all isolation devices. The expense of devices of this type prohibits their use in power supply applications.
From the foregoing, it can be seen that no device is known which provides a high degree of isolation at low cost, without requiring an optical isolator, a synchronizing signal or a complicated transformer structure to construct the isolated sensor.
Accordingly, it is an object of the present invention to provide an improved isolated sensor having a minimum number of components.
It is another object of the present invention to provide an improved isolated sensor which uses a transformer having a single primary and secondary winding.
It is still another object of the present invention to provide an isolated sensor which does not require synchronization between the circuitry on either side of the transformer.
It is yet another object of the present invention to provide an isolated sensor with improved bandwidth and stability over temperature and time.